April seems to have some special meaning for political extremists — and the dangerously disturbed. Some people associate it with Adolf Hitler’s birthday, April 20. It is an ominous month.

This year, on April 19, gun rights activists have scheduled a rally in Washington. In Oklahoma, some conservative legislators have proposed a volunteer state militia to resist federal government encroachments on state sovereignty. An organization called Oath Keepers is urging police, military, reservists and firefighters to pledge to refuse to obey “unconstitutional, unlawful orders.”

Meanwhile, some conservative websites show Democratic candidates in the cross hairs of a rifle. A Republican website that calls on voters to “Fire Pelosi” has a photo of House Speaker Nancy Pelosi (D-Calif.) engulfed in flames.

The theme of violent resistance is ugly. But as these April anniversaries remind us, it is not new. It has roots in some mainstream American values.

These days, extremist violence is associated mainly with fringe elements on the right, such as the militia movement that emerged in the 1990s. The country has not seen significant violence on the radical left since the Weathermen of the early 1970s.

Three deeply embedded forces appear to drive the temptation toward violence on the right.

One is distrust of government.

That is not new. The United States was settled mostly by people seeking religious or economic freedom. They were intensely distrustful of central authority. Distrust of government is written into our Constitution, with all its checks and balances and separation of power.

The belief in limited government is a mainstream American value.

Second is the gun culture.

Gun rights were written into the Bill of Rights. Pro-gun activists see the Second Amendment as giving citizens the right to defend themselves against a tyrannical government and to defend a system they believe is under threat.

“Over the last three or four years,” a tea party activist at an April 15 rally told The New York Times, “I’ve realized how immense that drift has been away from what made this country great.”

On the fringes of the right, defense turns into resistance. The implicit threat of violence is never far from the surface. “Violence is as American as cherry pie,” the left-wing black militant H. Rap Brown said during the 1960s.

D.H. Lawrence put it more poetically: “The essential American soul is hard, isolate, stoic and a killer.”

The notion of a conspiracy is never far from the surface either. “There’s a Communist living in the White House,” former “Saturday Night Live” comedian Victoria Jackson sang at the tea party rally.

“McCarthy Was Right” read one sign. Another read, “Hitler Gave Great Speeches, Too.” The ideologies may have been confused, but the point wasn’t.

Which leads to the third force — what historian Richard Hofstadter called “the paranoid style in American politics.” He described its hallmarks as “the sense of heated exaggeration, suspiciousness and conspiratorial fantasy.”